An alarm panel receives signals from a host of sensors and processes these signals to determine whether an alarm condition or other reportable event has occurred. The alarm panel is typically connected to a telephone line and uses this telephone line to report the alarm condition or a reportable event to a control station. The telephone line is normally shared by the alarm panel with other downstream telephone devices but the alarm panel has priority and the ability to interrupt communications of downstream devices.
Public telephone systems vary and different regulatory bodies have developed their own specifications that alarm panels and other devices must meet to be approved.
These different specifications are not easily met with a single cost effective circuit and different circuits have been developed to meet different regulatory specifications. Separate designs for each different market increases manufacturing costs and limits manufacturing flexibility. In France, for example, regulations require that the current draw of the device is limited to between 25 and 60 mA. This is in contrast to the U.S. and Canadian requirements which do not require the circuit to be currentlimited.
As will be more fully explained, an alarm panel switches between active and a passive state. In the active state, the alarm panel is using the telephone line to communicate with a remote computer to communicate, for example, with the owner at a remote location. In the passive state, the alarm panel monitors the telephone line for a ring signal and may also conduct tests to determine line integrity.
Control panels have previously used four or five optocouplers to electrically isolate the microprocessor of the alarm panel from the telephone line. It is also known to use only two optocopulers to isolate the alarm panel microprocessor from the telephone line; however, this 2 optocoupler design has serious compromises in the operating characteristics thereof.